1. Field of the Invention
The present invention relates to a shaft sealing apparatus for various kinds of pumps handling various low boiling point inflammable fluids such as butane, methane, ethane, propane, LNG (Liquefied Natural Gas) and etc.
2. Prior Art
Conventionally, in the case of pumps handling low boiling point and highly inflammable fluids such as butane, methane, ethane, propane, LNG and etc., it has been usual that a reservoir L is used so as to store and supply or pressure-supply a buffer solution or a sealing solution used for the mechanical seal M of a shaft sealing apparatus whereby a fluid leaking from the contact type seal is released outside (for example, into the flare) instead of being introduced into the atmosphere.
Further, there has been used a shaft sealing apparatus as described in Japanese Unexamined Patent Publication H6-42650 which has a favorable and stabilized sealing function in a high-pressure condition or with respect to a volatile or low boiling point fluid such as liquid ammonium.
However, the above-mentioned conventional shaft sealing apparatuses have had the following various problems. That is, where the above-mentioned reservoir L is used, the entire apparatus becomes large-sized or the manufacturing cost increases.
Further, since the gas in the purge fluid area flows into the reservoir L without any resistance, where a high-pressure sealing fluid leaks from the mechanical seal, the pressure of the purge gas as a sealing fluid for a dry gas seal (i.e., non-contact type mechanical seal) drastically changes in such a manner that it rises up immediately and then drops at once failing to enable a stable dry gas sealing operation to be carried out.
Moreover, where the purge gas discharge line is directly coupled to the flare, since harmful gases from other various facilities of the plant gather in the flare, there is a danger of the harmful gases concentrated into the flare entering into the shaft sealing apparatus. Therefore, when these harmful gases enter into the purge gas area of the sealing apparatus, they tend to leak from the sealing surface of the dry gas seal toward the atmosphere.
In addition, where the high-pressure sealing fluid leaks from the mechanical seal, the pressure of the purge gas as a sealing fluid of the dry gas seal immediately rises up so that the sealing fluid penetrates near the sealing surface of the dry gas seal. As the sealing fluid used in the sealing apparatus is highly inflammable and the dry gas seal also allows a small quantity of purge gas to leak, there is a danger of gas explosion at the bearings or drive device outside the sealing apparatus.
Lastly, in the case of the latter (i.e., the shaft sealing apparatus described in Japanese Unexamined Patent Publication No. H6-42650), since it lacks a purge gas supply system, the density of the sealing gas leaked into the purge fluid area becomes high so that the leakage of the high-density sealing gas into the atmosphere can not be prevented.